1. FIELD OF THE INVENTION
This invention relates to fuel elements for nuclear reactor cores and, more particularly, to apparatus for restraining longitudinal fuel element movement within a reactor core, and the like.
2. PRIOR ART
In a nuclear reactor, heat is generated for ultimate conversion into useful electrical power, and the like, through sustained fission processes that take place within the reactor core. To continue this sequence of nuclear fission events it is necessary to assemble a critical concentration of uranium, or other fissionable material, within the reactor core. Usually, an oxide of the fissionable material is shaped into small essentially right circular pellets that are located into long, slender hollow tubes, or fuel rods. Groups of two hundred or more of these fuel rods are arranged into more or less rigid fuel elements. These fuel elements, in turn, are organized into a large, generally cylindrical array that forms the reactor core.
Ordinarily, the collection of fuel elements that comprise the reactor core abut, on one end, a grill-like support grid that is disposed in a plane which is perpendicular to the longitudinal axes of the individual fuel rods.
To convert the fission process heat into useful work or into some form of usable energy, a suitable coolant is permitted to flow through the reactor core. Pressurized water is a typical coolant that frequently is used for this purpose. Most commonly, this pressurized water flows through the openings in the support grid and through the fuel elements in a direction that is generally parallel with the longitudinal axes of the fuel rods. It is, of course, from these fuel rods that the flowing pressurized water absorbs the heat the fission processes generate. After flowing through the fuel elements, the heated water is passed through a heat exchanger to transfer this heat to a secondary coolant, which is allowed to rise into the steam that drives the turbines in the power plant.
There is a natural tendency for the pressurized water to lift the fuel elements from the support grid. To cope with this particular hydrodynamic force, it has been customary to position another grid-like support plate at the discharge side of the reactor core, also in a plane which is generally parallel with the support plate at the inlet side. This discharge side support plate also is disposed in a plane that is perpendicular to the longitudinal axes of the fuel rods. To absorb and restrain longitudinal movement of the fuel elements, the ends of the fuel elements that are adjacent to the support plate at the discharge side of the reactor core usually are provided with coil springs which bear against the grid pads that protrude from the support plate toward the fuel elements.
Naturally, all of these fittings are not only very expensive but also tend to obstruct the flow of pressurized water through the reactor core, causing a reduction in the coolant flow volume, thereby producing further inefficiencies in the form of pressure losses.
After some period of use, a significant portion of the fissionable material within the core is consumed and the core, or a portion of the core, must be replaced. Frequently, the supplier for this replacement core is different from the original core supplier. There is, in this circumstance, a need to match not only the nuclear but also the hydraulic and mechanical characteristics of the replacement core to those of the original core. In this respect it is often necessary to conform the pressure loss of the replacement core to that of the original core without increasing or aggravating the hydrodynamic forces that tend to lift the fuel elements from the support plate on the inlet side of the core. There also is a further need, in many situations, to provide some means for resiliently absorbing or restraining the longitudinal motion of the fuel elements induced by the flowing coolant in a very small clearance between the ends of the fuel rods and the pads that protrude from the support grid at the reactor outlet.
Clearly, there is a need for an improved fuel element that will better restrain hydrodynamically induced fuel element movement at a lower cost than that which heretofore has been available, which will provide greater design flexibility for replacement (or "reload") core application, and the like.